. . . as an Austrian citizen, you can get a multi entry Tourist Visa only in the country where you have your registered residence. Or in the home country of your citizenship. You CANNOT get a multi entry Tourist Visa in ANY of Thailands neighbor countries. . . . . . . . . all you can get issued in Thailand's neighbor countries is a "Single Entry 60-days Tourist Visa". On most of the Thai embassies and consulates, you will need to make an appointment first. The only walk-in Thai embassy is the consulate in Savannakhet and Ho Chi Minh City
don't bother. Wait until the DLT issues a new license. What happened to me was, when I got my Thai licenses renewed for 5 more years, the DLT put the old passport number into the new licenses although I had given them my new passport with another number. You can't be bothered and the don't give a rat's A. Mai Ben Rai!
It is as it is - you can only apply for a multi entry Tourist Visa category in the home country of your citizenship. you could apply for in Cambodia if you hold a residence status in Cambodia. You mentioned you don't - which doesn't leave many options. You can but achieve up to 5 months stay 100% for sure, by entering with a 60 days single entry Tourist Visa, which you extend on Immigration for 1900.- THB for 30 more days. Then before 90 days are up you need to do a border run, this will give you a visa-exempt entry admitted 30 days, which you can also extend once, for 30 more days. This is already a months stay, and you could theoretically do a second border run giving you 30 days admitted stay, a last time. If you wish to be on the safe side, use "visa run companies" for these border runs, as they "grease the wheels"
if you ever plan to apply for the 1-year extension, make sure you got your marriage approved in Thailand. At the immigration, you will need a freshly printed Thai rose bordered marriage certificate from the Amphur
you don't need the 400,000.- THB in the bank for the VISA application. However you will need the 400.000.- THB deposit for the application to the 1-year EXTENSION
in order to apply for the "change of visa type" to a 90-days Non-Imm-ED visa, you will need all the relevant paperwork from the school ready to show Immigration. There is no guarantee that Immigration will accept it . . Check the requirements here, Your situation is in number 3, just click on it
5000.- THB is the standard price, and it goes without any tie-in deals like life-insurances or accident insurances, some banks want you to take up when you need a bank account with them. BEWARE!
of course there are agents in Bangkok, for the bank account and drivers license service. However I do not know any addresses, you have to google for them